1. Field of the Invention
The present invention relates to an efficient system for charging an energy storage capacitor such as that used to fire an electronic flash lamp. More particularly, it relates to such a system in which the capacitor is charged in stepped sequence to progressively higher voltage levels, so that the charging rate curve is a linear approximation of the optimum exponential charging curve of the capacitor.
2. Description of the Prior Art
High intensity electronic flash units have gained widespread acceptance for photographic applications, both as an integral component of the camera itself, or as an independent unit that is synchronized with the camera. In either case, a flash lamp is employed that produces an intense, short duration burst of light when appropriately excited. The flash is initiated by an ionizing pulse provided to a triggering element of the lamp. Upon ionization, a large burst of electrical energy must be supplied to the lamp, and this is provided by an energy storage capacitor which has previously been charged to a high energy level.
To provide portability, electronic flash units advantageously are battery powered. The lifetime of the battery, as measured by the number of lamp flashes that can be obtained before the battery must be replaced, is a function of the efficiency with which the energy storage capacitor can be charged. In prior art systems, such charging efficiency is low, typically on the order of forty percent. Indeed, for a system in which the charging current is limited to a particular maximum, the charging voltage as a function of time is a straight line. This means that the energy dissipated in the power supply initially is equal to the amount of energy transferred to the capacitor. Thus 50 percent efficiency is the maximum theoretically possible.
From an efficiency standpoint, it is most desireable to charge the energy storage capacitor along an exponential charging curve. Indeed, it is the principle object of the present invention to provide a system for charging a capacitor along a linear approximation of the optimum charge curve, thereby to obtain very efficient capacitor charging. Another object of the present invention is to charge the capacitor to sequentially stepped, increasing voltage levels in response to measurement of the actual voltage across the capacitor.
Some effort has been made in the past to accomplish capacitor charging in steps. An example is shown in the U.S. Pat. No. 3,654,537 to Coffey. There, a transformer has a plurality of secondary windings, each with a successively increasing number of turns. Separate bridge rectifiers are associated with each winding, and the outputs of these rectifiers are sequentially applied to the capacitor using a series of transistor switches coupled to a synchronizing firing circuit. While stepped charging of the capacitor is achieved, the circuit requires high voltage transistors to accomplish the switching. Moreover, the synchronizing circuit operates in binary counting fashion to apply progressively larger voltages to the capacitor, without the use of feedback to ascertain the charge level on the capacitor.
In other prior art systems, an inductive choke was included in the charging circuit to prevent excessive current surges, particularly at the beginning of the capacitor charging cycle. Such an inductive choke significantly increased the size and weight of the power supply, and made such power supplies inappropriate for light weight, portable uses. It is another object of the present invention to provide a light weight, efficient energy storage capacitor charging system which does not require an inductive choke to prevent excessive current surges.